In a conventional sawtooth wave generating apparatus, a signal having a desired reference frequency is generated using a fixed oscillation frequency of an oscillator such as a crystal oscillator, and then a sawtooth wave is generated using the signal having the desired reference frequency. The structure of a conventional sawtooth wave generating apparatus will be described with reference to FIGS. 9 and 10.
FIG. 9 is a schematic block diagram of a conventional sawtooth wave generating apparatus 2. As shown in FIG. 9, the conventional sawtooth wave generating apparatus 2 includes a fixed frequency generating section 3 for generating a reference frequency fixed at a predetermined value using a crystal oscillator or the like, and a sawtooth wave generating section 4 for generating a sawtooth wave based on the reference frequency. In this way, since using a piezoelectric element or the like having a piezoelectric effect as a load, i.e., driving a load whose resonant point is changeable was not taken into consideration, such the fixed frequency generating section 3 has been utilized.
FIG. 10 is a circuit diagram illustrating an example of the circuit structure of the sawtooth wave generating section 4 shown in FIG. 9. As shown in FIG. 10, the sawtooth wave generating section 4 includes a variable resistor 4R whose resistive value can be changed manually, and a plurality of capacitors 4C each having a predetermined capacitance, one of which can be set (selected). In this case, in the sawtooth wave generating section 4 shown in FIG. 10, the shape of the sawtooth wave including a slope of the sawtooth wave is changeable by changing the resistive value of the variable resistor 4R, and the cycle of the sawtooth wave (the period between a rising edge and trailing edge of the sawtooth wave) is changeable by selecting one capacitor within the plurality of capacitors 4C.
The generated sawtooth wave is utilized in an apparatus carrying out the PWM control such as a PWM driving device or a PWM display device as disclosed in, for example, Japanese Laid-Open Patent Application No. HEI. 11-180529. The PWM control is one in which a signal having a predetermined duty ratio for controlling the drive of each of various devices is utilized to control ON/OFF of the device by comparing a predetermined voltage value with the voltage value of the sawtooth wave (voltage waveform) by means of a voltage comparator.
Further, in Japanese Laid-Open Patent Application No. HEI. 11-180529, a method of controlling the drive of elliptic-oscillation parts feeder using a resonant frequency is disclosed in which the resonant point is track-controlled using a variable frequency power supply.
However, in the conventional sawtooth wave generating apparatus 2, in the case where the PWM control is carried out in a driving device using the resonant point of a resonant device, a clear (or stable) sawtooth wave could not be formed because the resonant frequency is changeable.
Namely, the sawtooth wave generated from the resonant frequency of a resonant driving device was utilized in the PWM control. However, since the resonant frequency was changeable, the PWM control could not be stabilized at a predetermined duty ratio. Thus, there is a problem that the duty ratio is changed constantly in such a case.
Further, there is another problem that a stable sawtooth wave having a certain shape cannot be formed which becomes a reference signal of the PWM control because the resonant frequency for the PWM control is changed constantly.